1. Field of the Invention
The present invention relates to a method and apparatus for measuring a surface reflectance (reflectivity) of a light-transmitting type antireflection optical function film and a method of producing a light-transmitting type antireflection optical function film by using the method of measuring the surface reflectance.
2. Description of the Related Art
Various display devices suffer from the disadvantage of a difficulty in viewing the content of the display due to reflection of light from the area around the display device, for example, light from fluorescent lights installed at the ceilings of offices or sunlight entering from windows.
Many attempts have been made to deal with this disadvantage. One approach has been to improve the method of lighting in the offices such as through the use of indirect lighting. Another approach has been to apply antireflection measures to the display devices themselves. The latter approach is necessary in cases where there is little incentive for making special improvements in the lighting system, for example, for television sets and other display devices installed in the general home. Of course, the same antireflection measures are effective for display devices used in offices as well, for example, computer displays.
As such display devices requiring antireflection measures, for example, cathode ray tube (CRT) displays, liquid crystal displays (LCD), plasma displays, electroluminescence (EL) displays, and RE displays. Below, the explanation will be given using a CRT display as a representative case.
Displayed information visible through the CRT display panel of the CRT display sometimes becomes difficult to see due to reflection of light from around the CRT display. Various antireflection measures against outside light have been devised for CRT display panels.
One of these antireflection measures against outside light is the provision of a light-transmitting type antireflection optical function film (hereinafter called just an "optical function film") on the surface of a CRT display panel.
The inventors previously proposed a "low reflection film and display panel using a low reflection film" in Japanese Patent Application No. 10-6999 filed on Jan. 16, 1998, in which they disclosed an example of an antireflection multilayer film which forms a surface of an optical function film suitable for such a purpose.
From the viewpoint of quality control of an optical function film and improvement of the efficiency of the production process, it is desirable to be able to accurately measure the surface reflectance of the optical function film in real time in the process of production of the optical function film and reflect (feed back) the results to the production process.
Measurement of only the surface reflectance of an optical function film per se has been difficult up to now, however, due to the effect of reflection from the back of the optical function film.
Therefore, up until now, the surface reflectance of an optical function film has been measured in a state of no reflection from the back of the optical function film by covering the back of a sample of the optical function film cut out from the process of production of the optical function film with for example a black paint absorbing the light.
Such a method of measurement, however, is a kind of destructive test of the optical function film since it is necessary to obtain samples of all of the optical function films for which the surface reflectance must be measured from the production process. Further, since the back of an optical function film for which the surface reflectance is to be measured must be painted black each time, time and trouble are taken. Further, this method of measurement of the surface reflectance involved manual work and therefore could not be applied to a process for continuous production of an optical function film where it is necessary to measure the surface reflectance of the optical function film in real time and notify the production line of the optical function film in advance and improve the production conditions when defective optical function film starts to be produced.
A method has been known for estimating the surface reflectance of an actual optical function film in a process of production of an optical function film by subtracting from the total reflectance measured in real time a reflectance correction value.
Further, a method has been proposed to calculate the surface reflectance of an optical function film from the actually measured total reflectance using a conversion formula.
Summarizing the disadvantage to be solved by the invention, with the above measurement methods, sometimes the surface reflectance obtained by the correction or calculation in the production process of the optical function film differed from the surface reflectance measured when painting black the back of a piece of the optical function film obtained by an actual sampling inspection, i.e., it was not possible to accurately measure the surface reflectance of the optical function film.
Further, in a sampling inspection of the optical function film etc., it was necessary to paint the back of a sample of the optical function film black to prevent reflection from the back of the optical function film. Also, that portion of the optical function film used for measurement of the surface reflectance was broken and could no longer be used.
Still further, with a sampling inspection, not all of the optical function film is inspected, so it was not possible to detect defects in parts of the optical function film. Accordingly, it is desirable to be able to accurately measure the surface reflectance of all of the optical function film continuously and in real time.
In addition, with the methods of the related art, since it was not possible to reflect (feed back) accurate measurements of the surface reflectance of the optical function film to the production process of the optical function film in real time, the disadvantage was encounted that the yield of the optical function film produced was low.
The above disadvantages will be discussed in further detail later with reference to the drawings.
While the above discussion was given with reference to measurement of the surface reflectance of an optical function film provided on a CRT display panel as the display panel, it is not limited to CRT display panels--similar disadvantages are encountered in optical function films provided on liquid crystal displays, plasma display, electroluminescence displays, RE displays, etc.